The present invention relates to a method and apparatus for vacuum decarburization refining a molten steel and, more particularly, to a method and apparatus, for refining a molten steel, that can inhibit the deposition of a splash onto the inner wall of a vacuum tank and an oxygen lance and at the same time can prevent oxidation loss of metal in the molten steel.
Conventional methods for additional decarburization refining of a molten steel which has been once subjected to decarburization refining in an electric furnace or a converter to provide a molten steel having a carbon concentration of not more than 0.01% by weight include: (1) a VOD (vacuum oxygen decarburization) method, typified by the one disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-43924, wherein an oxygen gas is blown onto the surface of a molten steel in a ladle while holding the molten steel surface in vacuo; and (2) a straight barrel type snorkel method wherein an oxygen gas is blown onto the surface of a molten steel within a snorkel submerged in molten steel to carry out vacuum refining.
In the method (1), VOD, a satisfactory space cannot be ensured above the molten steel surface. This causes a splash of molten steel, scattered during oxygen blowing decarburization refining, to be deposited onto a top-blown lance and a cover of a vacuum vessel, adversely affecting the operation.
The method (2), straight barrel type snorkel method, unlike the method (1), has no significant limitation on equipment, and an example of this method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912. The method disclosed in this publication is shown in FIG. 35. Specifically, in this method for vacuum refining of molten steel, a molten steel 71 contained in a ladle 70 is sucked through a snorkel 72 into a vacuum tank 73. An inert gas is blown into the molten steel within the snorkel 72 through under the plane of projection of the snorkel 72 within the ladle 70, and, at the same time, an oxidizing gas is blown through a top lance 74 onto the surface of the molten steel within the vacuum tank 73. In this case, the inner diameter of the snorkel 72 is determined so that the ratio of the inner diameter (D1) of the snorkel 72 to the inner diameter (D0) of the ladle 70, that is, D1/D0, is 0.4 to 0.8. In addition, the depth of blowing of the inert gas is determined so that the ratio of the depth (H1) of blowing of the inert gas as measured from the surface of the molten steel to the depth (H0) of the molten steel within the ladle 70, that is, H1/H0, is 0.5 to 1.0. The above method for vacuum refining of molten steel aims to efficiently carry-out decarburization without the deposition of the metal, slag and the like within the tank.
Japanese Unexamined Patent Publication (Kokai) No. 2-133510 proposes a vacuum treatment apparatus comprising: a ladle for placing therein a molten metal; a vacuum tank having a snorkel, submerged in the molten metal, provided at the lower end of the vacuum tank; an evacuation pipe connected to a vacuum source for evacuating the interior of the vacuum tank; and a shield disposed in the interior of the vacuum tank, wherein the shield is kept at a height of 2 to 5 m above the molten steel surface within the snorkel.
The method proposed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912, however, had the following problems (i) to (iv).
(i) Conditions for decarburization refining, such as the flow rate of the oxygen gas blown onto the molten steel, the flow rate of the argon gas for agitation, and the degree of vacuum within the vacuum tank 73, are not properly specified. This causes excessive fluctuation of the molten steel surface and splashing, leading to operation troubles attributable to deposition of the metal.
(ii) In the oxygen blowing decarburization refining of chromium-containing molten steel, such as stainless steel, the chromium component contained in the molten steel is oxidized with the blown oxygen. A part of the chromium oxide produced by the oxidation is reduced with carbon contained in the molten steel in the course of descending through the molten steel. Most part of the chromium oxide, however, undergoes the convection due to the inert gas blown from below the molten steel and floats, without being reduced, on the surface of the molten steel between the snorkel and the inner wall of the ladle to form slag 75 which is then discharged from the molten steel, increasing the loss of the chromium component.
(iii) The presence of the slag 75 containing chromium oxide causes the surface of the molten steel present between the snorkel 72 and the inner wall of the ladle to come into contact with air and to be cooled. This increases the viscosity of the molten steel surface. In addition, the slag 75, the metal or the like is deposited around the above inner wall of the ladle, making it difficult to conduct sampling of the molten steel in the course of and at the end of the refining, or making it difficult to move the snorkel 72 from the position of the ladle 70 at the end of the refining, which is an obstacle to refining.
(iv) The oxygen efficiency in the decarburization, defined as the ratio of the amount of the oxygen gas contributed to the decarburization of the molten steel to the total amount of the oxygen gas blown onto the molten steel, is influenced by refining conditions, such as the degree of vacuum in the vacuum tank 73, the state of agitation of the molten steel, and the flow rate of the oxygen gas blown. These refining conditions are not proper, making it difficult to maintain the oxygen efficiency in decarburization at a high level.
The method described in Japanese Unexamined Patent Publication (Kokai) No. 2-133510, wherein a shield is provided within a vacuum tank (a snorkel) to prevent splash of the molten steel created by oxygen blowing, thereby preventing deposition and accumulation of a metal caused by solidification of the splash deposited onto an oxygen lance, a vacuum tank, an evacuation pipe, had the following problems.
(i) When an exhaust gas is passes between shields within the vacuum tank, the molten steel splash in the exhaust gas or dust produced by solidification of the splash is deposited and accumulated onto the shields, increasing the flow resistance of the exhaust gas, which in turn increases the pressure loss within the vacuum tank.
(ii) Since the spacing, between the shields, serving as a passage for the exhaust gas becomes narrow, a high-power evacuation apparatus is necessary to provide a high degree of vacuum.
(iii) When a metal or the like scattered by splashing or spitting is once deposited and accumulated onto the passage for the exhaust gas between the shields, removal of the deposited and accumulated metal cannot be achieved without difficulty due to the complicated structure and requires a lot of time and labor.
In the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912, when the oxygen blowing refining is carried out at a high speed in order to increase the productivity of vacuum refining, the splashing is remarkably increased, posing the following problems which will be described with reference to FIG. 35.
(i) Although the creation of the splash of the molten steel 71 per se can be inhibited, dust is still contained in the exhaust gas. Therefore, the dust is gradually deposited within the evacuation duct 76 particularly around its duct inlet section to form a deposit 77, clogging the passage or increasing the air-flow resistance, which lowers the attainable level of the degree of vacuum within the vacuum tank 73.
(ii) Dust is introduced into a gas cooler 78 and damages the gas cooler. This results in suspension of equipment and increased maintenance cost. Further, a dust deposit is formed within the gas cooler 78, which causes a markedly lowered cooling efficiency.
(iii) Once a dust deposit 77 is formed within an evacuation duct 76, the dust is strongly united and must be manually removed. This increases the dust removal burden.
The method described in Japanese Unexamined Patent Publication (Kokai) No. 61-37912 is disadvantageous in that, for example, chromium oxide (Cr3O3) formed during oxygen blowing decarburization flows out from the snorkel into the outside of the vacuum tank and, since Cr2O3 has a high melting point, slag on the ladle is solidified, making it difficult to sample the molten steel, that is, posing a problem in the operation. An additional problem involved in this method is that Cr2O3, which has once flowed out into the outside of the tank, does not contribute to a later decarburization reaction, inevitably resulting in lowered oxygen efficiency in decarburization.
RH-OB is widely known as a method for oxygen blowing decarburization refining in vacuo. When this method is used, for example, in the finishing of stainless steel, aluminum is added to the molten steel before the oxygen blowing decarburization and combustion is carried out using top-blown oxygen to raise the temperature of the molten steel (aluminum temperature elevation or temperature elevation by aluminum). In this case, when aluminum temperature elevation is carried out under a high degree of vacuum, the depth of a cavity, of the molten steel, formed by a blown oxygen jet (cavity depth) becomes large, leading to a fear of bricks at the bottom of the tank being damaged by the blown oxygen jet, which makes it difficult to conduct temperature elevation by aluminum under a high degree of vacuum.
Further, the straight barrel snorkel type vacuum refining method is disadvantageous in that, as can be seen in the process for producing an ultra low carbon high chromium steel disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-43924, there is a limitation on the decarburization in a degassing period due to the difficulty of maintaining the agitating force and, as can be seen in the vacuum refining method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-305917, an attempt to improve the reduction rate in the degassing period results in remarkable wear of refractories.
Furthermore, after the oxygen blowing decarburization, introduction of aluminum as a reducing agent into the molten steel within the vacuum tank in order to recover a metal by reduction of a metal oxide, for example, chromium oxide, causes a rise in temperature of the molten steel by heat generated by thermit reaction, or scattering (bumping) of the molten steel or slag by a reduction reaction involving instantaneous evolution of CO gas, resulting in melt loss of refractories within the tank and deposition of the metal or slag, which is an obstacle to the operation.
A general object of the present invention is to solve the above problems created in oxygen blowing decarburization of a molten steel by the above-described RH-OB, VOD, or a refining method using a vacuum refining apparatus comprising a vacuum tank having a one-legged, straight barrel snorkel.
A more specific object of the present invention is to provide a method for vacuum decarburization refining of a molten steel that, even when the concentration of carbon in the molten steel is in a high concentration region, can inhibit the deposition of a splash onto the inner wall of the vacuum tank, the nozzle submerged in the molten steel, and the top-blown lance, prevent loss of a metal in the molten steel, for example, loss of chromium by oxidation, and, at the same time, reduce the fixation between the snorkel and the ladle by the slag.
Another object of the present invention is to provide means that does not increase flow resistance of an exhaust gas in a passage, shields the upper part of the vacuum tank and the oxygen lance from radiated heat during the vacuum decarburization refining, inhibits the entry of dust created by splashing of the molten steel into an evacuation system, and at the same time prevents clogging of the evacuation system with the dust.
A still another object of the present invention is to provide means that, during oxygen blowing decarburization in a high carbon concentration region, can prevent a metal oxide formed during the oxygen blowing decarburization from flowing out into the outside of the tank.
A further object of the present invention is to provide a method for adding aluminum that, at the time of raising the temperature using aluminum, can prevent the production of a metal oxide other than Al2O3 and the reposition of a large amount of the metal.
A still further object of the present invention is to provide a degassing method that can efficiently produce an ultra low carbon steel while preventing the production of a metal oxide in the molten steel.
The above various objects of the present invention can be attained by the following refining methods and apparatus.
At the outset, according to one aspect of the present invention, there is provided a refining method wherein a molten steel, which has been decarburized in a converter to regulate the carbon content to not more than 1% by weight (all xe2x80x9c%xe2x80x9d in the following description being by weight) is charged through a vacuum tank snorkel into a vacuum tank in a straight-barrel type vacuum refining apparatus; and in the vacuum tank, decarburization refining is carried out in such a manner that the carbon content of the molten steel is divided into a high carbon concentration region, which is a reaction region where the decarburization reaction rate is governed by the feed of an oxygen gas blown through a top-blown lance into the molten steel, and a low carbon concentration region which is a reaction region where the decarburization reaction rate is governed by movement of carbon in the molten steel, the degree of vacuum within the vacuum tank is regulated for each carbon concentration region and, at the same time, the flow rate of the oxygen gas blown through the top-blown lance is regulated to an optimal value (oxygen blowing conditions) for each carbon concentration region, and, in addition, the flow rate of an inert gas fed through a nozzle provided at a low portion of a ladle of the refining apparatus is also regulated for each region.
The above refining method can enhance the oxygen efficiency in decarburization and at the same time can prevent the occurrence of splash within the snorkel and the fixation of slag in the nozzle submersed portion.
Further, according to the present invention, at the time of oxygen blowing decarburization, particularly when a temperature elevation due to oxidation of aluminum (an aluminum temperature elevation in the following description being the same) is carried out, the degree of vacuum within the vacuum tank in the aluminum temperature elevation period, particularly in an oxygen blowing decarburization period in a region where the carbon concentration is not less than the critical carbon concentration region, is closely regulated according to the following conditions. This can prevent the deposition of the metal caused by splash or the oxidation of the metal.
Aluminum temperature elevation period: Gxe2x89xa6xe2x88x9220
Oxygen blowing decarburization period: xe2x88x9235xe2x89xa6Gxe2x89xa6xe2x88x9220
G=5.96-10xe2x88x923xc3x97Txc2x7ln(P/Pco)
wherein
Pco=760xc2x7[10(xe2x88x9213800/T+8.76)]xc2x7[%C]/[%Cr];
P: less than 760;
wherein
T: molten temperature, K; and
P: degree of vacuum within the tank, Torr.
For example, when the steel comprises 0.1% of carbon and 3% of chromium with the balance consisting of iron and T is 1700xc2x0 C., Pco is 1476 Torr. In this case, in order to regulate G to xe2x88x9220, P may be kept at 270 Torr. On the other hand, when the steel comprises 0.1% of carbon and 12% of chromium with the balance consisting of iron and T is 1700xc2x0 C., Pco is 370 Torr. In this case, in order to regulate G to xe2x88x9220, P may be kept at 67 Torr.
Introduction of aluminum and quick lime in an amount of 0.8 to 4.0 times the amount (kg) of aluminum added in the aluminum temperature elevation period and, in addition, introduction of a slag component, such as quick lime, in the oxygen blowing decarburization period in a high carbon concentration region to maintain the slag thickness at 100 to 1000 mm are also effective in preventing splash and in accelerating the softening of slag.
Further, the regulation of the depth of immersion of the snorkel in the molten steel in the aluminum temperature elevation period and the regulation of the immersion depth of the snorkel in the molten steel in the oxygen blowing decarburization period respectively to 200 to 400 mm and 500 to 700 mm can accelerate the reduction of a metal oxide (for example, Cr2O3 in refining of stainless steel) by a reaction with carbon contained in the steel, permitting the oxygen efficiency in decarburization to be kept on a high level.
According to the present invention, after the oxygen blowing decarburization, degassing is carried out under reduced pressure. In this case, an inert gas is injected from the low position of the ladle into the molten steel, of which the carbon concentration has been brought to around 0.01% by the oxygen blowing decarburization, in such an atmosphere that the degree of vacuum within the snorkel is in the range of from 10 to 100 Torr, so as to bring K value, defined by the following equation, to the range of from 0.5 to 3.5, thereby agitating the molten steel.
K=log{Sxc2x7Hvxc2x7Q/P}
wherein
K: agitation intensity at the activated surface;
S: activated surface area (plume eye area), m2;
Hv: depth of injected inert gas, m;
Q: flow rate of injected inert gas, Nl/min/ton-steel; and
P: degree of vacuum within the tank, Torr.
The degassing treatment can maintain the renewal of the interface at a activated surface, which is a substantial gas/metal reaction interface, enabling a high-purity molten steel having an attained carbon concentration of not more than 10 ppm to be effectively produced.
When introduction of aluminum for reduction, after the degassing treatment, to reduce a metal oxide (for example, Cr2O3 in the case of refining of stainless steel) produced during oxygen blowing, thereby recovering the metal, is necessary, an inert gas for agitation is injected into the molten steel in the flow rate range of from 0.1 to 3.0 Nl/min/ton-steel (in terms of flow rate per ton of molten steel to be refined; hereinafter referred to as xe2x80x9cNl/min/txe2x80x9d) in an atmosphere having a low degree of vacuum of not more than 400 Torr, or alternatively, it is possible to employ a method wherein, immediately after the degassing treatment, the pressure is returned to the atmospheric pressure, the vacuum tank is lifted, and, simultaneously with the lifting of the tank, aluminum for reduction is introduced into the molten steel and an inert gas for agitation is injected into the molten steel at a flow rate of 0.1 to 3.0 Nl/min/t during the introduction of aluminum for reduction and at a flow rate of 5 to 10 Nl/min/t after the introduction of aluminum for reduction. The injecting of the inert gas by the above method can prevent a rapid rise in temperature of the molten steel or bumping of the molten steel and at the same time can prevent nitrogen pickup in the reduction period.
The present invention provides a vacuum decarburization refining apparatus that can inhibit the deposition of splash (droplets) created by splashing or bumping, or dust formed by solidification of the splash onto the inner wall of the vacuum tank and the snorkel submerged in the molten steel, which is a major problem to be solved by the invention. The vacuum decarburization refining apparatus has the following construction.
At least one burner is provided on the side wall, in an upper tank, in the vicinity of the canopy of the vacuum tank, and a space having a larger inner diameter than the inner diameter of the snorkel is provided in a lower tank in the vacuum tank. In addition, a shielding section, which has at its center a space having an inner diameter smaller than each tank and larger than the outer diameter of the top-blown lance, is provided, between the lower tank and the upper tank at a position which receives enough radiated heat to melt the deposited metal, integrally with the side wall of the vacuum tank.
The vacuum tank having the above construction permits the influence of a high temperature, around a hot spot created by the blowing of oxygen through the top-blown lance and the decarburization reaction, on the refractories in the side wall of the lower tank to be avoided, and at the same time enables the metal deposited on the shielding section to be melted by radiated heat. Further, dust, constituted by splash which has ascended to the upper tank without being deposited onto the shielding section and has been deposited in the vicinity of the canopy, is melted by means of the burner, flows downward and is removed.
Further, the evacuation duct disposed between the vacuum tank and a gas cooler for cooling an exhaust gas comprises an ascendingly inclined section inclined upward from an duct inlet provided in the upper tank of the vacuum tank and a descendingly inclined section inclined downward from the top of the ascendingly inclined section. Therefore, splash of the molten steel and dust, which, together with an exhaust gas, have entered the evacuation duct are collected in a dust pot provided below the descendably inclined section without being deposited within the evacuation duct.
As described above, a major object of the present invention is to increase the oxygen efficiency in decarburization while minimizing splash, bumping and other unfavorable phenomena created in the course of refining. Since, however, means is provided which, even when splashing or the like is created, can effectively avoid or remove droplets or dust derived from the splashing and the like, the degree of vacuum within the vacuum tank can be always kept on a desired level, realizing stable operation.